This invention relates to apparatus for calcining fine grained material such as cement raw meal, limestone, dolomite, and the like. The invention is specifically directed to an apparatus for manufacturing cement clinker employing a suspension preheater, a precalcining furnace, a clinkering furnace and a clinker cooler, but it should be understood that it can be applied to uses which do not include a separate clinkering furnace.
Prior to the present invention, suspension preheaters which utilize the exhaust gases from the clinkering furnace or rotary kiln of a cement clinker manufacturing facility were well known. Also known prior to the present invention was the use of a precalcining furnace or flash calciner positioned at the material outlet of the suspension preheater and before the clinkering furnace. The use of a precalcining furnace serves to perform most of the raw material calcining function outside of the rotary kiln so that the rotary kiln has the limited function of completion of calcination and the primary function of sintering the calcined raw material to form cement clinker. This use of a precalcining furnace has enabled a substantial increase in production to result and, in fact, permits substantially a two-fold increase in the production capabilities of a given size rotary kiln.
Typically, a flash furnace will receive preheated air for combustion from the clinker cooler of the cement manufacturing plant. This may be supplied to the flash furnace either from a separate duct connected directly from the cooler vent outlet to the flash furnace, or excess air may be supplied through the rotary kiln to be burned in the flash furnace. If desired, the exhaust gases from the rotary kiln may be mixed with cooler exhaust air at a point external to both the rotary kiln and the flash furnace to then be supplied to the flash furnace. Typical of the prior art is U.S. Pat. No. 3,891,382 of which the present inventor is a co-inventor.
It is known that there are instances when it is a disadvantage to utilize too many stages of preheat of the raw material and the gases discharged from the preheater are at a substantially elevated temperature. One example is where the raw material includes a carbon bearing material such as oil shale. If such a material is exposed to a gradual heating from a relatively low temperature to a higher temperature in a multiple stage preheater, the carbon bearing material will reach a temperature at which the carbon will volatilize. This can result in a plugging of the preheater. For this reason, conventional preheaters have not found acceptance in the cement industry where carbon bearing raw materials are encountered. If such a raw material is utilized, it is advantageous to use either a single stage of preheat or no preheater at all. In such an installation, the temperature of the exhaust gases from the preheater, if there is one, or the exhaust gases from the kiln itself are quite high and often wasted. Prior to the present invention it has been known to use such gases in a waste heat boiler. It has been found by the present invention that this heat can be utilized in the cement manufacturing operation by using this heat to heat the combustion gas supplied to a precalcining furnace.
Another application where it is not practical to use prior apparatus for manufacturing cement which includes a flash furnace is the application where preheated air from the clinker cooler cannot be utilized in the precalcining furnace. One such instance is where an attached tube clinker cooler is employed. Such coolers are unable to provide a volume of preheated air which can be transported to the flash furnace separate from the preheated air supplied to the kiln.
Another instance where waste heat from a cooler is unavailable for use in a precalcining furnace is in the manufacturing of white cement. In the manufacture of white cement, it is possible that no clinker cooler will be used. If a cooler is used, very little if any recoverable heat may be available from the cooler. In addition, white cement plants are often operated at high temperatures resulting in large quantities of high temperature gases being discharged from the preheater. In the above applications some other source of preheated combustion air must be found in order to utilize the full advantages of a precalcining furnace and make such an installation fuel efficient.